This invention relates to alarm systems and, more particularly, to an alarm system wherein a central monitoring station is sequentially connected to poll a plurality of remote transponder units, each located at a respective protected premises.
There are presently in existence many different types of alarm systems for monitoring selected conditions at a protected premises and providing an alarm indication upon detection of an abnormality in the selected condition. For example, conditions which are typically monitored are the presence of a fire, the opening of a door or window which should be in a closed state, a holdup attempt as signaled by the pushing of a "panic" button, etc. Such alarm systems range from the relatively simple to the very elaborate and sophisticated.
A simple alarm system would utilize sensor elements, such as fire detectors, which, when actuated, would cause the sounding of an audible device such as a loud beeper or horn on the protected premises. Such a system functions solely as a local warning that an abnormal condition has occurred, and no one off the protected premises is aware of such. In the event that an abnormal condition occurs while no one is on the premises, such as a fire or break-in during the night, it would be desirable to provide an indication of such to persons who will take action in response thereto, i.e. the local fire department or police department.
Different systems have been developed to accomplish the foregoing objective. For example, one way of accomplishing the objective is to have apparatus which is responsive to an abnormal condition for dialing and setting up a telephone connection between the apparatus at the protected premises and a preselected telephone at for example the local fire department or police department, the apparatus including a prerecorded message which is repeated a predetermined number of times after the telephone connection is established. Such a system has a number of disadvantages and thus is not entirely satisfactory. For example, the described system can be rendered inoperative by disconnecting the power or telephone line. In either event, a message cannot leave the protected premises.
A system which is in widespread use and obviates the above disadvantages includes a central monitoring station having dedicated wires connected between the central monitoring station and all the different protected premises. At the protected premises, the various conditions sensors are connected to the dedicated wires so that when all conditions are normal, a closed loop is formed between the central monitoring station and the protected premises over the dedicated pair of wires. When a sensed condition is abnormal, the loop is opened by the particular sensor. The central monitoring station is responsive to the opening of the loop to provide an alarm indication at the central monitoring station. If power is lost at the protected premises or if the wires are cut, this will open the loop and provide an alarm indication at the central monitoring station. Although this system is an improvement over the local alarm system, it also suffers from several disadvantages. In particular, a burglar can short the loop and prevent any alarm indication from being given. Also, using only a single pair of wires in the manner described, it is not possible to distinguish the type of abnormal condition.
In order to overcome these disadvantages, various other more sophisticated and elaborate systems have been devised. For example, U.S. Pat. No. 3,256,517 discloses a system wherein a plurality of remote stations are all connected to a single central station. The central station transmits a combination of tones to all the remote stations. Each remote station is responsive to a unique predetermined tone combination so that the central station can selectively address a desired remote station. The remote station responds only to the combination of tones comprising its address to retransmit to the central station the same group of tones if none of the alarms at the remote station have been actuated. If the alarm function corresponding to a particular tone of the address group has been actuated, this tone is shifted in frequency, and the shifted tone is retransmitted as part of the tone group in place of the normal tone of that group. The central station is thus made aware that an alarm condition has occurred and further which alarm condition has occurred. A particular disadvantage of this system is that it requires dedicated wires between the central monitoring station and all the different remote stations.
U.S. Pat. No. 3,209,342 discloses a system wherein, upon the occurrence of an alarm condition at a remote station, a first frequency signal is sent to a central station. The central station then sends an interrogation signal to the remote station. A further frequency signal is then transmitted from the remote station to the central station to indicate the particular alarm. This system also requires dedicated wires from the central station to the different remote stations. Furthermore, as this system requires the remote station to initiate action, it is subject to tampering.
U.S. Pat. No. 3,725,865 discloses a system including a central station and a large number of remotely located stations. The central station contains a plurality of receivers, each corresponding to a different remote station and each tuned to the frequency of the associated remote station. When there is an alarm condition at a remote station, a signal of the assigned frequency is transmitted to the central station. This system suffers from the same disadvantages as the last-described system.
It is therefore an object of this invention to provide an alarm system wherein a central monitoring station monitors alarm conditions at a plurality of remote stations, each located at a respective protected premises.
It is a further object of this invention to provide such an alarm system wherein, when an alarm condition occurs, the central monitoring station can detect the type of alarm condition.
It is another object of this invention to provide such an alarm system wherein the absence of a signal indicates an alarm condition so that tamper protection is provided.
Many types of protected premises require different condition sensing dependent upon whether the particular premises is in an occupied or unoccupied state. For example, during the time when the premises is occupied (during the day) certain doors such as the front door are repeatedly opened and closed. During unoccupied times, these doors should remain closed. For practical purposes, the door condition sensor should remain in place at all times, but the sensing of the door opened condition should only cause an alarm to be generated during certain time periods. It would therefore be desirable to provide means for conditioning the remote station to inhibit the sensing of certain conditions during certain times. This means could include a switch which may be selectively placed in the DAY position during occupied times of the premises and in the NIGHT position during unoccupied times. The first person to enter the premises could then move the switch from the NIGHT position to the DAY position and the last person to leave the premises could move the switch from the DAY position to the NIGHT position. However, if an alarm condition exists, such as a door or window being ajar, it would be desirable to prevent the last person from leaving the premises until that condition is corrected.
It is therefore yet another object of this invention to provide an alarm system as described wherein means are provided to condition the system to respond to certain alarms only during certain times.
It is still another object of this invention to provide such an alarm system which may be manually transferred from a day monitoring condition to a night monitoring condition and wherein when such transfer is attempted, the person attempting such transfer is notified if an alarm condition exists so that such person may correct such condition.
Typically, the dedicated wires between the central station and the associated remote station are leased from the local telephone company. The cost of such dedicated leased lines is a major cost of the alarm system, and such cost is expected to increase dramatically. The Bell System is currently introducing DATAPHONE (a registered service mark of AT & T Company) Select-A-Station Service which is a private line data service designed for applications in which a master station exchanges voiceband data information with a number of remote stations, one at a time. Point-to-point voiceband connections are set up between the master station and each remote station to allow this exchange of information. The service allows two-way transmission between the master station and the remote station, but no direct transmission is available between remote stations. Connection control can be achieved only from the master station. The security of this service makes it particularly well suited for alarm central station applications. In particular, the fact that connection control can come only from the master station and the fact that all remote stations other than the one connected at a particular time are isolated from the connected path and from each other ensures that no trouble on one remote leg can affect proper cooperation of the remainder of the circuit. This isolation of each point-to-point connection also ensures the privacy of communication between the master station and each remote station.
It is therefore an additional object of this invention to provide an alarm system as described which is compatible with the above-described Bell System Select-A-Station Service.
Any alarm system compatible with the Bell System Select-A-Station Service must be capable of polling the remote stations at spaced intervals to ensure that no alarm condition goes undetected for more than a minimal time.
It is therefore still another object of this invention to provide such an alarm system as described wherein the central monitoring station sequentially and repetitively polls the plurality of remote stations.